How awful. I do not have good advice, I do not live anywhere near you.
I do however want to say I too have had a doctor who said it wasn't the MG. I'm seropositive (and young female) too.
I went to another, and a little later another.

My EMG always turns out normal, even when I'm in a (near) crisis. It just is, I do not know why. But the neurolist I have now, says EMG can help if they're positive, and are useless if they're negative. It just means thát specific (group of) muscle(s) isn't affected at thát specific moment. Oh hurray, my thumb muscles are good...pity I can't walk, eat or breath....but who cares: the EMG is normal. It's all in my head.
Anyone can see that is not the way to handle this.

I truly hope your daughter will get the help she needs, and soon.

Anyway, apart from this, if it even ís a conversion disorder, that's a serious and awful disease too and should be treated as such.